Optical nonlinearities of nanostructured VO2 thin films with low phase transition temperature

Vanadium dioxides (VO2) thin films which change from a monoclinic semiconductor phase to a tetragonal metallic structure at the temperature of 29 °C have been fabricated by reactive ion beam sputtering. Micrograph of scanning electron microscope (SEM) shows that the grain size of VO2 crystallite ranges from 20 nm to 50 nm. Regeneratively amplified Ti:sapphire laser pulses were applied to induce the phase transition, which was accompanying with the third-order optical nonlinearities in VO2 thin films. Open-aperture and closed-aperture measurements of Z-scan were used to study the optical absorptive and refractive nonlinearities. Nanostructured VO2 thin films exhibit two-photon absorption and a negative nonlinear index of refraction when phase transition is induced. The optical nonlinearities are due to excitation of electronic subsystem only and without involving of the structural semiconductor-to-metal phase transition.